This invention relates generally to frequency response control in an audio system having a speaker for emitting an audio signal to a sound field, and more particularly to a circuit arrangement which corrects frequency response in accordance with frequency response or acoustic characteristic of a sound field.
Frequency response of an audio system has hitherto been controlled by way of a graphic equalizer so that a desired frequency response results. When it is intended to take the frequency response of a sound field, such as a listening room, into account, the frequency response determined by the acoustic characteristic of the sound field is first measured, and necessary correction or compensation is then effected. Although such control can be manually effected, it is time-consuming and troublesome, and therefore automatic frequency response control circuits have recently developed.
One example of such a known automatic equalizer/analyzer is disclosed in a publication from AUDIO ENGINEERING SOCIETY, which includes information presented at the 64th Convention, Oct. 31-Nov. 3, 1980 in New York. According to the technique disclosed by Robert W. Adams of dbx, Inc. a digitally controlled 10-band equalizer is used where the equalizer comprises 10 filters and 10 D/A converters. Namely, the equalizer requires a plurality of filters and D/A converters whose number equals the number of bands to be controlled. The equalizer is controlled by a computer which is required to output band select data of 4-bit and level control data of 8-bit, where these data are fed to the D/A converter of the equalizer in parallel bit form. Therefore, the number of connecting lines between the computer and the equalizer is large, while the structure of the equalizer is complex. In addition, when an equalizer is constructed by a plurality of band pass filters, distortion is apt to occur due to phase variation.
Another prior art reference, i.e. Japanese Patent Provisional Publication No. 56-17507 (Application No. 54-93498) published on Feb. 19, 1981, discloses a digitally controlled signal control device in which an input audio signal is level controlled by a digital signal formed by 16-bit parallel data word. In this way according to known techniques it is necessary to process a relatively long data word of a number of bits, and therefore the structure of a level adjusting circuit or equalizer has suffered from complex structure. Such complex structure results in high manufacturing cost, while it is difficult to construct a main portion of the equalizer by way of an integrated circuit.